Kaposi's sarcoma (KS) is a mutifocal vascular tumor of mixed cellular compositions that is the most common neoplasm in patients with AIDS. Kaposi's sarcoma-associated herpesvirus (KSHV) has been consistently identified in KS, primary effusion lymphoma (PEL) and some forms of Multicentric Castleman's disease (MCD). The presently available information provide compelling evidence that KSHV is the long sought infectious cause of KS, PEL, and MCD. The proposed research is directed toward investigating the molecular mechanism of cell growth control and viral gene expression by the Kaposi's sarcoma-associated virus (KSHV) viral interferon regulatory factor 1 (vIRF1) gene product. As seen with growth deregulating proteins of other tumor viruses, vlRF1 has pleotropic effects on cellular signal transductions. Our preliminary studies have found that vlRF1 interacts with the p300 transcriptional cofactor and p53 tumor suppressor and these interactions inhibit their biological activity. Additionally, vlRF1 recruits the viral protein kinase ORF36 and RNA polymerase (Pol) II and its expression enhances the phosphorylation of RNA Pol II carboxyl terminal domain (CTD). Based on our preliminary results, we hypothesize that vlRF1 contributes to KSHV persistent infection and pathogenesis by inhibiting p53 tumor suppressor to facilitate cell growth deregulation, by altering p300 activity to suppress host IFN-mediated anti-viral activity and cellular gene expression, and by interacting with the ORF36 protein kinase and RNA Pol II to activate viral gene expression. In this proposal, we will draw upon our experience in the biochemical analysis of viral gene function and in the development and utilization of expression and genetic systems to define the detailed mechanisms of cell growth deregulation and viral gene expression by vlRF1. Our biochemical and cell biology studies will define in greater detail the molecular mechanisms used by vlRF1 to inhibit p300 and p53 activity. To define the role of vlRF1 in viral gene expression and replication, we will utilize an inducible viral gene expression system and KSHV Bacmid. Because of the lack of cell culture systems, the combination of both systems provides a unique opportunity to investigate the contribution of vlRF1 to KSHV gene expression and replication. The proposed studies will provide an understanding of a novel pathogenic strategy of KSHV to survive destruction by host immune attack and to deregulate cell growth control.